Deuterium -- an isotope of hydrogen with an extra neutron -- is a key substance in understanding how the elements were created during the first few minutes of the universe. This process, known as Big Bang nucleosynthesis, is believed to have fixed the ratio of deuterium to hydrogen (D/H) that we see in the universe today. It also created the primordial "baryonic soup", consisting of light atoms from which stars emerged.

According to the Caltech researchers, the D/H ratio can be calculated by measuring how the CMB radiation was absorbed by the neutral gas that existed during the "cosmic dark ages", which began roughly 400,00 years after the Big Bang. Hydrogen and deuterium absorb CMB photons at different wavelengths, and the technique would involve looking for tiny fluctuations in those absorptions caused by variations in the density of the two gases at different points in space. For example, regions of the Universe with a higher density of hydrogen atoms will absorb more CMB photons than regions of lower density.

Because the D/H ratio is very small, seeing the individual correlations between these wavelengths in a given pixel will not be possible, says Steven Furlanetto, who carried out the study with Kris Sigurdson. "However, if we average over many pixels on the sky it may be possible to see the overall correlation strength, and this number is simply proportional to D/H," he adds.

Until now, cosmologists have only been able to determine the D/H ratio indirectly. They have done this by using the standard model for Big Bang nucleosynthesis together with constraints on the total density of the nuclei as measured from the CMB by satellites such as NASA's Wilkinson Microwave Anisotropy Probe. The new technique would instead allow the ratio to be measured directly using radiotelescopes.

The Caltech team claims their technique would allow the primordial D/H ratio to be determined to an accuracy of better than 1%. This, in principle, would improve the constraints on the baryonic density of the Universe and could shed more light on the nature of non-baryonic dark matter. "The key advantage of our approach is that it’s direct and model-independent," says Furlanetto.

Moreover, other direct measurements for D/H are all made after stars and galaxies have formed. "Since deuterium is destroyed by stars, our method of measuring deuterium before the stars formed is the only way to be 100% sure you are observing the primordial deuterium abundance," adds Furlanetto

One of the main obstacles facing any experimentalist seeking to take up the challenge will be background noise. This interference comes from terrestrial sources, distortion from the ionosphere (which absorbs and refracts the cosmological signal) and other astronomical sources – such as the galactic synchrotron background.

"Beating down the noise and these huge backgrounds will require much more powerful telescopes and probably more sophisticated data analysis algorithms," Furlanetto concludes.